Some notes from overhauling many of these over the years:
1) The corrector should be centered in its cell (using the shims), but ONLY if the primary mirror is centered in the tube.
I used to assume the primary was centered, but on some SCTs, it wasn't (!). The corrector must be centered on the primary, so check that first.
2) The secondary should be centered within the corrector. Since the secondary mirror may not be accurately centered in its holder, you cannot tell this
unless the corrector and secondary are removed to note the offset if there is any. The secondary holder may need to be a bit eccentric in the corrector if the secondary mirror
is off center. If the secondary is more than a millimeter off-center, I'd seriously contemplate detaching it and reattaching it accurately centered.
3) The secondary should also be centered on the primary (see note #2) but ONLY if the primary baffle is in the tube completely straight, i.e. is parallel to the tube.
You cannot assume that it is. I have seen many scopes where the primary baffle did NOT point at the secondary mirror exactly and one side of the baffle clipped
the light cone more than the other. The difference of a millimeter isn't important, but I had a couple that had to be returned to the manufacturer to align the primary baffle because
the movement of the primary mirror was not coincident with the line of the tube wall due to a tilted baffle.
4) It is unlikely the exact center of the secondary will reflect a laser beam back at its source when/after the scope has been star-collimated.
Even the tiniest of tilt corrections in the secondary due to its being a trace off-center will return a laser beam to a different point. This is one reason NOT to use a laser for
secondary collimation. The daylight collimation test http://www.robincasa...ro/collimation/ only works if everything is centered and lined up. Otherwise, it merely gets you
close enough to use a star for collimation. Checking collimation on a star (artificial or real) is critical for best results.
5) The inside surfaces of correctors all develop a haze after a while and need to be cleaned. This is easiest if the corrector + secondary is removed and placed back in the scope backwards.
Then the inside is easy to clean.
6) Rotation of the corrector relative to the primary should be maintained unless you're willing to experiment. Slight eccentricities in secondary placement sort of demand it.
If the corrector and cell do not have registration marks under the pressure ring that holds the corrector in place, make your own with a Sharpie pen or white-out right at the edge of the corrector.
The mark should be small, so it will be hidden by the pressure ring when it is replaced.